The present invention relates to a susceptor for mounting and holding a wafer substrate in a plasma excitation CVD equipment. More specifically, the present invention relates to a surface structure of the same and a process for producing the same.
A CVD equipment in which a thin film is grown on a wafer substrate in a vacuum container having a vacuum exhaust mechanism using a source gas excited with a RF plasma has been generally used in a procedure of producing a semiconductor and a liquid crystal display. A wafer substrate is mounted on a susceptor constituting one side of a RF electrode, and heated to a predetermined temperature of approximately 400.degree. C. for obtaining a high-quality film grown. A source gas fed to the vacuum container is introduced into a plasma space formed between an upper electrode to which a RF power is supplied and a lower electrode earthed, and is excited actively.
When silicon dioxide (SiO.sub.2) is formed on a wafer according to a plasma excitation CVD method, a monosilane (SiH.sub.4) gas and a dinitrogen monoxide (N.sub.2 O) gas are used as a source gas. When silicon nitride (SIN) is formed on a wafer substrate, a monosilane (SiH.sub.4) gas, an ammonia (NH.sub.3) gas and a nitrogen (N.sub.2) gas are used as a source gas.
In the step of forming a film on a wafer substrate, a film is also formed on a position other than a substrate inside a vacuum container, for example, on a surface of a susceptor which is situated outside of the surface of the upper electrode or the periphery of the wafer substrate. There is a problem that the thickness of the undesirous film is gradually increased, and this film is peeled off owing to the change in the temperature of the surface, floats within the reactor, and is then dropped on and adhered to the wafer substrate. In order to prevent this phenomenon, the inside of the reactor is periodically cleaned in the conventional plasma excitation CVD equipment through plasma etching to remove the undesirous film. In the cleaning of the inside of the reactor through plasma etching, a fluorine-containing gas such as CF.sub.4, C.sub.2 F.sub.6 or NF.sub.3 is generally used as a source gas. A fluorine-containing active ion seed chemically removes products adhered to an electrode surface, a susceptor surface or an inner wall of a reactor.
A conventional susceptor is generally formed of aluminum or an aluminum alloy, and its surface is machined smoothly up to an average roughness (Ra) of 1 .mu.m or less. The Ra value here refers to an arithmetic mean roughness, and it is an index value of a roughness defined in JIS BO601-1994. The larger the Ra value, the coarser the surface.
Since a luminum is liable to be reacted with fluorine and there is a possibility of by-product contamination, an anodized film for protection from a fluorine plasma is coated on the surface of the susceptor as described in U.S. Pat. No. 5,039,388.
However, this conventional susceptor structure involves the following problems. As indicated by Solid State Technology, p. 139, April 1990 by H. P. W. Hey et al., a wafer substrate mounted on a susceptor having an anodized surface is to be placed in an electric field generated in a plasma space, a nd undergoes ion impact therein, with the result that the wafer substrate is charged in the plasma treatment within the reactor and adsorbed on the susceptor. When the wafer substrate is adsorbed on the susceptor after the film-forming, the substrate cannot easily be removed from the susceptor, making it impossible to smoothly practice the subsequent substrate treatment and the cleaning treatment of the inside of the reactor through plasma etching which is conducted after carrying out the substrate.
Therefore, raised and depressed portions are formed on the surface of the conventional susceptor through shot blasting. This processing is conducted to decrease a rate of contact between the susceptor surface and the reverse surface of the wafer substrate and reduce an adsorption force.
However, in this conventional method, a steep protruded tip is formed on a surface of a susceptor. This tip is repeatedly brought into contact with the wafer substrate, and gradually worn out. Solid particles formed therein are one cause of impurity contamination.
The wear of the tip also gives a serious influence on qualities of a thin film formed besides the contamination. When the wear proceeds, the rate of contact between the susceptor and the wafer is increased. Accordingly, heat conduction from the susceptor to the wafer is changed, and the wafer temperature deviates from the set value. When the wafer temperature is higher than the set value, the growth rate of the film is decreased, and a film having a small thickness is formed within a predetermined film-forming period of time. When the wear proceeds and the rate of contact between the susceptor and the wafer is increased, the potential of the wafer substrate approaches that (zero potential) of the earthed state of the susceptor. Consequently, the ion impact from the upper RF electrode is increased, and a hard film having a high compressive stress is formed. Thus, the wear of the steep tip formed on the surface of the susceptor gives a serious influence on process conditions of the film.